It has long been known that providing an endplate on the wing tip of an aircraft airfoil reduces induced drag at the wing tip. Induced drag is caused by the difference in fluid flow pressure on the upper and lower surfaces of the airfoil. At the wing tip, this pressure differential produces fluid-flow that is transverse to the stream of lift-producing fluid; that is, because the fluid pressure is higher below the wing than above, fluid flows from below the wing to above the wing around the wing tip in a direction transverse to that of the lift-producing fluid. The endplate on the wing tip impedes this transverse fluid flow and thus reduces the induced drag on the airfoil.
These endplates are relatively thin fins or shields that extend streamwise along the wing and project upwardly to impede transverse fluid flow along the top of the wing. Non-planar structures, such as tubular members with circular or elliptical cross-sections, have also been mounted on wing tips to reduce induced drag by impeding transverse fluid flow. The above-described endplates and other non-planar structures, while generally effective at reducing induced drag in low speed aircraft, were inappropriate for use with modern high speed aircraft because on high speed aircraft they tended to increase profile drag, interference drag, compressibility drag, flow separation at the wing tip/endplate intersection, and/or the bending moment at the root of the wing. Therefore, endplates and other non-planar structures have not been successfully employed on modern high speed aircraft.
For these and other reasons, designers began to apply the same aerodynamic principles that govern wing airfoil design to the design of endplates. Such aerodynamically efficient endplates will be referred to herein as winglets. These winglets are able to reduce induced drag while lowering profile drag, interference drag, compressibility drag, and/or the bending moment at the root of the wing. However, it is well-known that these winglets are still susceptible to flow separation at the wing/winglet intersection.
More particularly, the flow separation at the wing/winglet intersection is caused by interference between the fluid flowing over the upper surface of the wing and the fluid flowing over the upper surface of the winglet. The fluid streams flowing over these two upper surfaces diverge as they pass over the wing and winglet upper surfaces between the thickest portions and the trailing edges of the wing and winglet. These diverging streams of fluid will be discussed in more detail below with reference to FIGS. 3 and 4. Flow separation occurs in the areas on the upper surfaces between these divergent streams of fluid. This flow separation is particularly acute at low speed, high lifting conditions.